This invention relates to apparatus and methods for an ion source that comprises a flow-through ultraviolet lamp for production of ions for analysis by, for example, mass spectrometry.
Mass spectrometry is an analytical methodology used for quantitative and qualitative chemical analysis of materials and mixtures of materials. In mass spectrometry, a sample of a material, usually an organic or inorganic or biomolecular sample, to be analyzed called an analyte is broken into electrically charged particles of its constituent parts in an ion source. The particles are typically molecular in size. Once produced, the analyte particles are separated by the spectrometer based on their respective mass-to-charge ratios. The separated particles are then detected and a mass spectrum of the material is produced. The mass spectrum is analogous to a fingerprint of the sample material being analyzed. The mass spectrum provides information about the masses and, in some cases, quantities of the various analyte ions that make up the sample. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments of an analyte. Additionally, to some extent mass spectrometry can identify molecular structure and sub-structure and components that form the structure within the analyte based on the fragmentation pattern when the material is broken into particles. Mass spectrometry has proven to be a very powerful analytical tool in material science, chemistry and biology along with a number of other related fields.
Mass spectrometers employing ionization chambers, such as atmospheric pressure chemical ionization (APCI) chambers, have been demonstrated to be particularly useful for obtaining mass spectra from liquid or gaseous samples and have widespread application. Mass spectrometry (MS) is frequently used in conjunction with gas chromatography (GC) or liquid chromatography (LC), and combined GC/MS and LC/MS systems are commonly used in the analysis of analytes having a wide range of polarities and molecular weights. LC/MS systems have been particularly useful for applications such as environmental monitoring, pharmaceutical analysis, industrial process and quality control, and the like.
APCI may be used in conjunction with gaseous or liquid samples. In APCI-MS, in one preferred operating mode, a liquid sample containing mobile phase (solvent) and analyte is converted from liquid to vapor phase, followed by ionization of the vapor and analyte. Such systems frequently employ nebulizers, usually based on pneumatic, ultrasonic, or thermal xe2x80x9cassistsxe2x80x9d, to break up the stream of liquid entering the nebulizer into fine, relatively uniform-sized droplets, which are then vaporized. Ionization of the vaporized mobile phase and analyte molecules occurs under the influence of a corona discharge generated within the APCI chamber by an electrically conductive corona needle to which a high voltage electrical potential is applied. In APCI with liquid samples, the mobile phase molecules serve the same function as the reagent gas in chemical ionization mass spectrometry (CIMS). The mobile phase molecules are ionized by passing through a high electric field gradient or corona discharge created at the tip of the corona needle (electrode). The ionized mobile phase molecules then ionize the analyte molecules. The exact chemical reactions and resulting ions depend upon the composition of the mobile phase, whether APCI is operated in positive or negative mode, and the chemical nature of the analyte. More than one type of ion may be formed, leading to multiple mechanisms for ionization of the analyte. A fraction of the ionized analyte and solvent molecules is separated from vaporized and non-ionized solvent molecules and is subsequently focussed and analyzed by conventional mass spectrometry techniques.
Atmospheric pressure photoionization (APPI) utilizes a source of ultraviolet (UV) to ionize molecules of interest in mass spectrometry. One commonly used source is a plasma induced discharge (PID) lamp. These lamps consist of a cylindrical glass bulb filled with a noble gas such as argon, krypton, xenon, and the like. A plasma is induced in the gas via a radio frequency (RF) coil wrapped around the glass bulb, which is opaque to UV radiation. UV radiation emitted by the plasma is transmitted through a window bonded to one end of the glass cylinder. Typical window materials used to transmit the UV radiation are magnesium fluoride, calcium fluoride, lithium fluoride and so forth.
The UV radiation emitted by these lamps in the range useful in a mass spectrometer source (100 nm to 150 nm) is absorbed by air, water vapor and many of the solvents used in mass spectrometry over a short path length of a few mm. Therefore, it is necessary to locate the UV source very close to the vapor stream from the MS vaporizer. The standard PID lamp configuration often does not provide efficient illumination of the vapor molecules to produce the abundance of analyte ions desired. It is therefore desirable to provide much greater illumination of the vapor molecules.
One embodiment of the present invention is an ion source comprising (i) a vapor source, which produces a directed stream of vaporized molecules within the ion source and (ii) an apparatus for conducting photoionization within the ion source. In one embodiment the apparatus comprises a tubular outer element, a tubular inner element and a source of an electrical field. The tubular inner element is disposed within the outer element to provide a space between the outer and inner elements. The tubular inner element is open at its ends to provide a pathway therethrough. Creation of an electric field between the two elements generates a discharge in a gas contained in the space between the elements producing ultraviolet radiation, which transmits through the wall of the inner element into the space within the inner element. Molecules flowing through the space within the inner element are ionized by the UV radiation.
Another embodiment of the present invention is a method for ionizing molecules in an ion source. Vaporized molecules are flowed through a region within the ion source. UV radiation is generated to surround the vaporized molecules flowing through the region, thereby ionizing a portion of the vaporized molecules.